In applications where very high strength-to-weight ratios are desired, such as in helicopter rotor blades, aircraft structural components, re-entry vehicles, heat ablators, gas turbine components, and the like, composite materials are of particular utility. Typically, a composite material comprises a reinforcement which can be continuous or discontinuous in a matrix of another material, for example, graphite fibers in a thermosetting plastic resin.
The principal advantages of the composite are the combination of desired properties from the substituent materials and the designer's ability to tailor resulting properties to meet the needs of a specific application. The composite owes its effectiveness to the fact that the reinforcements and matrix bond quite readily. In the above-given example, graphite contributes to the high tensile strength of the composite, while the matrix is stronger in compression than in tension.
A major problem that has arisen in this technology is the lack of uniformity in the quality of the cured composite. Further, variability, whether batch-to-batch or within batch, is difficult to detect and makes quality control a formidable task. Thus, there exists a need for methods which reduce and/or control the factors that lead to variability in the quality of the composite material during the critical stages of manufacture.
One factor which affects batch quality is the heat transfer characteristics of the oven/autoclave employed to cure the composite parts. Typically, the autoclave is employed to drive a batch of composite parts through a specific temperature profile or cure cycle as recommended by the material suppliers or as developed by the material users. However, oftentimes, the heat transfer characteristics of the autoclave will vary from location to location within the autoclave itself. Such variations, whether due to the particular stacking configuration of the parts within the autoclave or the autoclave characteristics, result in inconsistent batch quality.
Heat transfer, as used herein relates to the transfer of energy or heat by virtue of a temperature difference. There are three modes of heat transfer: conduction, convection and radiation. The present invention is particularly directed at heat transfer by convection wherein energy is exchanged between a solid surface and an adjacent fluid, usually in motion.
There are at least two ways in which fluid motion can be produced in a heat transfer situation. There is forced convection wherein an external agent such as a pump or fan is employed to circulate the fluid. Additionally, there is natural convection wherein the fluid is set in motion by the buoyant force resulting from density changes in the fluid caused by temperature differences therein. The present invention is concerned with both of the above-mentioned types of convection heat transfer.
In order to access the heat transferred between a fluid and a solid surface, one must know the convective heat transfer coefficient h which is defined as follows: ##EQU1## wherein Q/A is the heat flow per unit surface area
T.sub.s is the surface temperature, and PA1 T.sub.oo is the free-field temperature of the fluid (i.e. outside the boundary layer). The convective coefficient h represents a relative conductance between the solid surface and the fluid. The coefficient varies with fluid properties, flow geometry, and temperature level.
Variations in the heat transfer characteristics of the autoclave can affect batch quality due to the inherent weakness of certain crosslinking polymers, such as many thermosetting plastic resins, during and after gelation. Because gelation is usually followed by vitrification, a partially cured resin has virtually no strain capability. Thus, if the recommended cure cycle is not properly instituted, a real possibility of matrix fracturing exists. This damage is permanent, and the result, in the cured composite, is microcracking or delamination or both. A further possibility is weak interfacial bands between the matrix and reinforcements caused by the imposition of intolerable shear stresses on the resin after gelation and prior to a complete cure.
Currently, air temperature heating cycles for oven/autoclaves are derived or generated through trial and error techniques in order to drive a part or batch of parts through a specific cure temperature profile. When the oven/autoclave heating loads are varied to an extent which results in a deviation from the anticipated heat transfer characteristics from the air to the part, bad parts result. Thus, there exists a need for a heat transfer monitoring device for use during the cure processing of composite parts.